If the installation is successful, you will be prompted to run the command:

% brew doctor

The Terminal will then provide the status of your Homebrew install. If Homebrew is not ready to go, common issues include outdated XQuarts and outdated git. The first can be remedied by downloading the latest XQuartz from the page linked above, while the former simply needs the following command:

% brew install git

Once Homebrew is ready, the command: brew doctor, will return "Your system is raring to brew."

Installing Drush (using http://drupal.org/project/drush)

Drush is a PHP component available through the PEAR distribution system and can therefore be installed over the PEAR channel with the following commands: